The invention relates to applying coating color onto a fiber web and especially to curtain and jet coating of a fiber web and to a method for coating of a fiber web in a fiber web production line. More especially the invention relates to a coating device having a nozzle unit with at least one nozzle part having a feeding chamber and at least one equalizing chamber, a feed hole between the feeding chamber and the first equalizing chamber and a nozzle slot, from which the coating color discharged through an outlet opening of the nozzle slot and to a method for coating in which method the coating color is applied by said coating device.
As known from the prior art in fiber web producing processes typically comprise an assembly formed by a number of apparatuses arranged consecutively in the process line. A typical production and treatment line comprises a head box, a wire section and a press section as well as a subsequent drying section and a reel-up. The production and treatment line can further comprise other devices and sections for finishing the fiber web, for example, a sizer, a calender, and a coating section. The production and treatment line typically also comprises a reel-up and at least one winder for winding customer rolls as well as at least one roll packaging apparatus. In this description and the following claims by fiber webs are meant, for example, paper and board webs.
In the production of fiber webs, for example of paper or board webs, sizing is used to alter the properties of a fiber web by adding sizing agents (sizing medium), for example glue chemicals. Sizing can be divided into internal sizing and surface sizing. In internal sizing the sizing agent is added to pulp in the wet end of the fiber web machine before forming. In surface sizing the sizing agent is added onto the surface of the fiber web at the dry end of the fiber web machine.
In the production of fiber webs, for example of paper or board webs, in coating, especially the surface of a fiber web is formed with a layer of coating color (coating medium) at a coating station followed by drying. The formation of a coating in direct coating applications can be divided into supplying the coating color onto the web surface, which is called the application of the coating color, as well as in the adjustment of the final amount of coating color. In indirect coating applications the adjustment of the color amount is controlled already when supplying the color.
The coating of a fiber web typically utilizes a coating device—a coater—, which together with, for example drying devices following the coater, forms the coating section of a fiber web production line. In connection with the coaters different kinds of application technology for application of the coating medium on the fiber web are employed in prior art arrangements, for example curtain technology or blade application technology or rod application technology or air brush application technology or spray application technology. The present invention relates to curtain application technology, which is one of the most important coating techniques. Curtain coating is suitable to coat different types of coated paper and board grades. By curtain coating a good coverage of coating color on the fiber web surface to be coated is achieved. The present invention also relates to jet coating.
Coater equipment based on applying the coat to the surface of a moving web by means of an unguided jet directed to the web surface are generally known in the art as jet nozzle applicators. In these jet nozzle applicators, the coating color is applied to the web surface with the help of a separate jet-forming slot nozzle, whereby the equipment construction may be varied widely. In one type of equipment construction the coating color is directed to the web surface as a narrow linear jet via a nozzle slot extending over that portion of the cross-machine width of the web which is to be coated. This type of a coater is also known as a fountain coater. In the jet nozzle coater, or the fountain coater, the entire amount of required coating is transferred to the web surface. The application of the coating is performed to the surface of the web running supported by a backing roll, and conventionally, the coat is smoothed immediately after application by means of a doctor blade adapted to the perimeter of the same backing roll.
In EP patent publication 0838551 discloses an assembly for coating a moving web of paper or paperboard, said assembly comprising an applicator for applying a coating color to the web in the form of a linear jet ejected via a narrow-gap slit orifice adapted to extend at least over the portion of the cross-machine width of the web to be coated. The assembly comprises means for gauging the coat weight applied to the web at least in the cross-machine direction and control elements for altering the amount of coating color in the jet, which is ejected from the nozzle slot at multiple points along the cross-machine width of the web.
In curtain coating two main types of curtain coating devices are used, namely curtain coating devices with slot-fed and curtain coating devices with slide-fed.
In the slide fed curtain coating devices, coating color is fed by means of a nozzle assembly onto an inclined plane and the coating color flows down towards an edge of the plane constituting a feeding lip and the curtain is formed as the coating color falls off the feeding lip. In the slot-fed curtain coating devices coating color is pumped through a feeding chamber into a narrow vertical slot and the curtain is formed at its lip and falls onto the web. Coating can be applied in one or more curtain layers. The curtain is maintained at full width by means of an edge guide which is located along the edge of the feeding slot/feeding lip.
WO patent application publication 2005/024132 discloses a paper/board web coating device which is arranged to extend in its longitudinal direction in the transverse direction of the web to be coated, and which comprises a nozzle unit having at least one feeding chamber extending in the longitudinal direction of the coating device, into which feeding chamber is conveyed coating color by feeding means, and a nozzle slot in flow communication with the said feeding chamber, the said nozzle slot also extending in the longitudinal direction of the coating device, and to which nozzle slot the coating color is supplied from the feeding chamber over the total longitudinal distance of the nozzle slot and further conveyed out of the outlet opening of the nozzle slot and the flow communication between the said at least one feeding chamber and the nozzle slot connected to it is formed by feed holes made in one wall of the feeding chamber, through which the coating color can be conveyed to the nozzle slot, and the nozzle unit has means by which the effective area of the feed holes can be adjusted in order to accomplish transverse profiling of the amount of coating color. In this device according to prior art between the said at least one feeding chamber and the nozzle slot connected to it is at least one equalizing chamber which also extends in the longitudinal direction of the coating device and into which equalizing chamber the feed holes open and as an adjustment means for the area of the feed holes, a profiling member is arranged in the equalizing chamber located on the surface of the equalizing chamber comprising the feed holes, extending over a length determined by successive feed holes and being adjustable for changing the effective area of individual feed holes or groups of several feed holes for feeding the desired amount of coating color into the equalizing chamber at different points of its longitudinal direction. As an adjustment means for the area of the feed holes in this prior art device is also suggested an adjusting pin connected to each feed hole respectively, which pin is movable in its longitudinal direction for changing the effective area of each feed hole as desired.
FI patent publication 118926 discloses a paper/board web coating device which is arranged to extend in its longitudinal direction in the transverse direction of the web to be coated, and which comprises a nozzle unit having at least one feeding chamber extending in the longitudinal direction of the coating device, into which feeding chamber is conveyed coating color by feeding means, and a nozzle slot in flow communication with the said feeding chamber, the said nozzle slot also extending in the longitudinal direction of the coating device, and to which nozzle slot the coating color is supplied from the feeding chamber over the total longitudinal distance of the nozzle slot and further conveyed out of the outlet opening of the nozzle slot and the flow communication between the said at least one feeding chamber and the nozzle slot connected to it is formed by feed holes made in one wall of the feeding chamber, through which the coating color can be conveyed to the nozzle slot. In this device according to prior art between the said at least one feeding chamber and the nozzle slot connected to it is at least one equalizing chamber which also extends in the longitudinal direction of the coating device and into which equalizing chamber the feed holes open. The feed holes between the feeding chamber and the equalizing chamber are formed in a replaceable, separate part with a desired flow opening.
In prior art devices for curtain or jet coating the coating color, the application beam comprises a feeding chamber into which the coating color is supplied typically either through an opening located in the middle of the application beam in the cross direction or through an opening located at one end of the application beam. In order to create a desired feeding profile of the coating color the coating color is fed from the feeding chamber to the equalizing chamber through a relatively small feed hole before feeding the coating color through the nozzle slot. The coating color flow in the feeding chamber is laminar-turbulent, turbulent in the feed holes and in the equalizing chamber and in the nozzle slot laminar.
One disadvantage in these kinds of prior art arrangements is that in the small feed holes the shearing rate of the coating color varies, thus causing variations in coating profile. During the passing through the feed holes the coating color is affected by different rates of shearing depending on the location of the path of the coating color at the cross section of the feed hole. Viscosity of coating color does not recover immediately to follow the original viscosity curve but instead the recover curve depends on the maximum shear that influenced the coating color in the feed hole. In particular in connection with coating colors that are shear-thickening at least at some rate value area of shearing rates, viscosity behavior of coating color will be problematic after the feed hole when the shearing rate decreases to low values. Thus in practice the coating color when entering the equalizing chamber has no more homogenous viscosity and thus the coating color distributes non-uniformly due to different flow resistance, which causes variation in feeding profile flow in the nozzle slot. This causes streaks of the coating color on the fiber web and even very sharp stripes in the coating color amount.